Oil sand, such as is mined in the Fort McMurray region of Alberta, Canada, generally comprises water-wet sand grains held together by a matrix of viscous bitumen. It lends itself to liberation of the sand grains from the bitumen, preferably by slurrying the oil sand in heated water, allowing the bitumen to move to the aqueous phase.
For many years, the bitumen in McMurray oil sand has been commercially recovered using a hot water process well known in the art. Generally, oil sand is mixed in a tumbler with hot water having a temperature of approximately 80-90° C., steam, caustic (e.g., sodium hydroxide) and naturally entrained air to yield a slurry having a temperature typically around 80° C. The slurry so produced is diluted with additional hot water to produce diluted slurry having a temperature of about 65° C. to about 80° C. The diluted slurry is introduced into a large, open-topped, conical-bottomed, cylindrical vessel termed a primary separation vessel (PSV) where the more buoyant aerated bitumen rises to the surface and forms a froth layer.
However, while the hot water process assured good bitumen recoveries for all grades of oil sand, the thermal energy requirement per tonne of oil sand processed for the steam production and for heating hot flood water is very high.
Recently, in an attempt to reduce the thermal energy requirement for bitumen extraction from oil sands, a low energy extraction process or the “LEE process” for bitumen extraction was developed, which is generally described in Canadian patent No. 2,217,623 and U.S. Pat. No. 6,007,708. The LEE process generally comprises the following steps:                dry mining the oil sand;        mixing the mined oil sand with water in predetermined proportions near the mine site to produce a slurry containing entrained air and having a controlled density in the range of about 1.4 to about 1.65 g/cc and preferably a temperature in the range of about 20° C. to about 50° C. or higher;        pumping the slurry through a pipeline having a plurality of pumps spaced along its length, preferably adding air to the slurry as it moves through the pipeline, to condition the slurry (i.e., ablating the larger lumps of oil sand to release bitumen and allowing the bitumen flecks to coalesce and attach to air bubbles);        diluting the conditioned slurry with flood water and introducing the diluted slurry into a primary separation vessel (PSV) to float the aerated bitumen and separate it from the middlings and tailings (primary tailings). The froth is maintained at a temperature of at least 35° C. in the PSV by use of a heated water underwash to optimize separation of the bitumen. Primary tailings, primarily comprising coarse solids, water, and residual bitumen, which settle to the bottom of the PSV and secondary tailing primarily comprising fines, water, and residual bitumen, which are produced from the further processing of the PSV middlings in flotation cells to remove bitumen still remaining in the middlings, are disposed of accordingly.        
While the thermal energy requirements of the hot water process are significantly reduced in the LEE process, nevertheless, thermal energy in the form of heated flood water is still required for slurry preparation, slurry dilution and for the PSV underwash to ensure the overall PSV slurry temperature of at least 35° C.
Finding sources of thermal energy for the LEE process, however, becomes problematic as oil sands mining and extraction operations are being located at considerable distances away from upgraders such as cokers, which are an economical source of thermal energy. These satellite oil sands operations still require considerable supplemental heat input to achieve the targeted bitumen recoveries. Thus, the heat input comes predominantly from natural gas delivered through a gas turbine operated with auxiliary burning as well as by utilizing natural gas fired auxiliary boilers.
Currently, both the heat (thermal energy) and any residual bitumen present in the primary and secondary tailings are lost in the tailings deposition process. In fact, using optimum LEE process conditions still results in only about 90 to about 94% bitumen recovery depending on the ore blend, pipeline conditioning and recycle water chemistry. Thus, it would be beneficial, both from an energy conservation and an improved bitumen recovery point of view, to capture the heat and bitumen in tailings.
Thus, there is a need for a process that can be used for both bitumen and heat recovery from oil sand tailings.